multiple contacts merged into one. message conversations still split

Hi my girlfriend just got a new SIM, and that caused our conversations to be split into three conversations.

1. her old number which sends both texts and imessages (formerly the only message thread we had)

2. her new number which only texts

3. her icloud which can only send imessages

ive already merged them all into one contact. how do i get them all into one thread?

devices: mine-iphone 6, ios11 . hers- iphone 5, ios 11